File:Xlib square example.png

/*
  * Simple Xlib application drawing a box in a window.
  * gcc input.c -o output -lX11
  */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include <X11/Xlib.h>

int
main(int argc, char **argv)
{
        Display *display;
        Window window;
        XEvent event;
        char *msg = "Hello, World!";
        int s;

        /* Open connection with the server. */
        display = XOpenDisplay(NULL);
        if(display == NULL){
                fprintf(stderr, "Cannot open display\n");
                exit(1);
        }

        s = DefaultScreen(display);

        /* Create window. */
        window = XCreateSimpleWindow(display, RootWindow(display, s), 10, 10,
                200, 200, 1, BlackPixel(display, s), WhitePixel(display, s));

        /* Select kind of events we are interested in. */
        XSelectInput(display, window, ExposureMask | KeyPressMask);

        /* Map (show) the window. */
        XMapWindow(display, window);

        /* Event loop. */
        for(;;){
                XNextEvent(display, &event);

                /* Draw or redraw the window. */
                if(event.type == Expose){
                        XFillRectangle(display, window, DefaultGC(display, s),
                                20, 20, 10, 10);
                        XDrawString(display, window, DefaultGC(display, s), 50,
                                50, msg, strlen(msg));
                }

                /* Exit on key press. */
                if(event.type == KeyPress)
                        break;
        }

        /* Close connection to server. */
        XCloseDisplay(display);

        return 0;
}